Although numerous studies point to a critical role for intravascular volume as sensed by cardiovascular receptors in the control of water intake, the central neural mechanisms responsible remain obscure. Recent data indicate that pontine neurons, especially in the ventral aspect of the locus coeruleus and in the parabrachial nuclei, respond to decreases in intravascular blood volume, and that electrolytic lesions of the region antero-ventral to the locus coeruleus lead to a marked hyperdipsia. However, it is not clear whether these pontine neurons play a role in hypovolemic thirst. This proposal is designed to explore the role of neurons in the ventral aspect of the locus coeruleus and in the parabrachial nuclei in hypovolemic induced thirst in freely behaving rats, by using electrolytic and neurotoxin lesions. In addition, this study is designed to examine the role of these pontine neurons in the control of basal water ingestion and urinary water and electrolyte excretion. Together, these experiments will provide critical preliminary data for a role of pontine neurons in the mediation of hypovolemic induced thirst and in the attenuation of thirst after drinking.